Bottom hole assembly (BHA) whirl and bit whirl are two types of downhole vibration that can severely damage drilling bits and downhole tools. “Whirl” is used to describe the rotational motion of a bit, BHA, or the drill string itself, in which the bit, for example, is rotating at a different rotational velocity with respect to the borehole wall than it would be rotating if the bit axis were stationary. This movement may be faster, or slower, than the case where the bit axis is stationary. In forward whirl, the drill string rotates clockwise, which is similar to the rotation of the bit on its axis. In backward whirl, the drill string rotates counter clockwise against the bit rotation axis.
When drilling a well and after a new connection is made, usually the driller will start up the mud pumps and rotate the top drive before the bit touches the bottom. The top drive will rotate to the RPM set point, which will rotate the BHA and bit. So far, there is not much load or weight put on the drill string. If the well is slightly deviated from vertical, that creates a mass imbalance and if the RPM set point is close to the lateral resonant frequency, then the BHA will start to forward whirl. After that, the driller will start to add weight and start drilling. Most of the time, forward whirl will disappear with the added weight. In severe situations where backward whirl is excited, the only way to mitigate the backward whirl is to pick up off bottom and completely stop rotating and then restart.
Efforts to reduce whirl have included modeling whirl vibrations, designing bits and the BHA to reduce whirl, detecting whirl using downhole sensors, and changing drilling parameters to reduce whirl. For example, when sensors indicate that vibration levels have exceeded some safe level, the WOB or RPM are adjusted.
Most of the solutions concentrate on designing drilling bits that reduce whirl and/or on BHA design by adding stabilizers in the right locations along the BHA that helps maintain stable rotation of the drill string relative to the wellbore. These physical solutions, however, are costly, and if they do not work, one has to wait until the next bit trip to make any modifications to the BHA or bit.
Thus, what are needed are systems, apparatuses, and methods that provide more effective ways to control forward whirl conditions.